Screw compressors have been used as compressors for compressing a refrigerant or air. For example, Japanese Patent Publication No. H06-042474 discloses a single screw compressor including a screw rotor, and two gate rotors.
The single screw compressor will be described below. The screw rotor is substantially in the shape of a column, and a plurality of helical grooves are formed in an outer peripheral surface thereof. The screw rotor is contained in a casing. The helical grooves of the screw rotor constitute fluid chambers. Each of the gate rotors is substantially in the shape of a flat plate. The gate rotor includes a plurality of rectangular plate-shaped gates which are radially arranged. The gates of the gate rotor mesh with the helical grooves of the screw rotor. When the screw rotor is rotated, the gates move relatively from the start ends (ends through which the fluid is sucked) to terminal ends (ends through which the fluid is discharged) of the helical grooves, and the fluid is sucked into the fluid chambers for compression.
A screw compressor disclosed by Japanese Patent Publication No. H03-081591 includes a lubrication passage for supplying lubricant oil to the fluid chambers. In the screw compressor disclosed by Japanese Patent Publication No. H03-081591, a sump for collecting the lubricant oil is formed in the casing, and the lubricant oil in the sump is supplied to the fluid chambers due to difference in pressure between the sump and the fluid chamber. The lubricant oil supplied to the fluid chamber is used to lubricate the screw rotor sliding on the casing, and to seal between the screw rotor and the casing to ensure gastightness of the fluid chambers. The lubricant oil supplied to the fluid chamber is used to cool the fluid compressed in the fluid chamber, or the screw rotor.